The present invention relates to a method of manufacturing methanol, particularly, to a method of manufacturing methanol in which carbon dioxide is utilized for increasing the methanol yield.
Japanese Patent Disclosure (Kokai) No. 1-180841 discloses a method of manufacturing methanol (CH.sub.3 OH) from hydrocarbons such as natural gas by the processes described below:
(Synthetic Gas-Forming Process)
In the first step, a synthetic gas containing as main components hydrogen (H.sub.2), carbon monoxide (CO) and carbon dioxide (CO.sub.2) is formed in a reformer by the reaction between a natural gaseous hydrocarbon or a gaseous hydrocarbon evaporated from a liquid hydrocarbon and steam at a predetermined temperature in the presence of a nickel-based catalyst.
Steam is added to the hydrocarbon from a moistening device arranged upstream of the reformer, followed by supplying a superheated steam prepared in a boiler or the like to the hydrocarbon so as to introduce a gaseous mixture containing hydrocarbon and steam into the reformer.
Since the steam reforming reaction noted above is an endothermic, reaction accompanied by a large amount of reaction heat, the reformer is heated from the outside in the process of forming the synthetic gas.
(Crude Methanol Synthetic Process)
A crude methanol is synthesized by the reaction among the synthetic gas, carbon monoxide and hydrogen or among the synthetic gas, carbon dioxide and hydrogen at a predetermined pressure and temperature in the presence of a methanol synthesizing catalyst to synthesize a crude methanol.
(Distilling Process)
The liquid crude methanol recovered in the methanol synthesizing process is distilled in a single or a plurality of distillation columns so as to separate the crude methanol into a waste liquid material and a refined methanol, said waste liquid material containing organic compounds having melting points lower than that of methanol (hereinafter referred to as "low boiling point organic compound"), organic acids and organic compounds having boiling points higher than that of methanol (hereinafter referred to as "high boiling point organic compound").
Methanol is manufactured via the processes described above.
In recent years, it is of high importance to suppress the amount of carbon dioxide discharged from a plant as a measure against warming of the earth.
In a plant for manufacturing methanol from hydrocarbon such as natural gas, the heat required for the endothermic reaction between carbon monoxide and hydrogen is supplied to the reformer by heating the reaction tube loaded with a steam reforming catalyst with a combustion gas. Also, a boiler for steam generation is used for replenishing a required amount of a high pressure steam consumed in the plant. Naturally, a large amount of carbon dioxide is contained in the combustion waste gas of the reformer and the boiler for the steam generation. It follows that the plant tends to be rendered disadvantageous in economy where a tax and regulation for the carbon dioxide discharge are started in future.
On the other hand, in the method of manufacturing methanol from natural gas, the hydrogen concentration in the synthetic gas formed by the steam reforming reaction is about 1.5 times as high as that required for synthesizing methanol by the reaction of hydrogen with carbon monoxide and carbon dioxide contained in the synthetic gas. Therefore, in the process of synthesizing methanol, the unreacted gas separated from the synthesized methanol is recycled to the synthesizing reactor in order to improve the reaction efficiency within the synthesizing reactor, and the unreacted gas is partly released to the outside of the system to release the excess hydrogen. Also, the recycling amount of the unreacted gas is set at a value which permits moderating the heat generation rate during reaction in the catalyst layer loaded in the synthesizing reactor.
Under the circumstances, the idea of supplying carbon dioxide to a fluid passageway through which a synthetic gas formed in a reformer is supplied to the methanol synthesizing reactor is shown in FIG. 5 of "INCREASED PRODUCTION FROM EXISTING METHANOL PLANTS BY A. English, I. A. Forbes, M. N. Islam, J. D. Korchank PRESENTED TO: WORLD METHANOL CONFERENCE Dec. 2-4, 1991 HYATT REGENCY HOTEL VANCOUVER, BC, CANADA, pp. 1-12".
However, if a synthetic gas containing a large amount of carbon dioxide is supplied to the reactor in the crude methanol synthesizing step, the activity of the methanol synthesizing catalyst loaded in the reactor tends to be lowered.